The present invention relates to the preparation of zinc carbonate, and in particular, to the preparation of zinc carbonate from zinc ash using urea.
A substantial amount of zinc metal ends up as skimming or ash during the galvanizing process. In addition, there are other waste zinc materials from die casting and electric steel furnaces. All these zinc materials are contaminated to some extent with one or more of the following impurities: iron, lead, other heavy metals, aluminum, manganese, etc.
Zinc ash is traditionally used to make zinc chemicals which include zinc chloride and zinc sulphate. The markets for these chemicals are limited with little growth. The traditional method of using zinc sulphate and soda ash to make zinc carbonate or active zinc oxide from zinc ash is cumbersome and expensive.
For example, U.S. Pat. No. 4,071,357 teaches a process of recovering zinc value from steel making flue dust. In this process, a solution of ammonia and carbon dioxide is used to dissolve the zinc bearing flue dust.
One disadvantage with the method disclosed in U.S. Pat. No. 4,071,357 is that shipping and storing ammonia and carbon dioxide is costly and involves high pressure and/or low temperature. Further, transportation of ammonia and/or carbon dioxide in small amounts tends to be expensive as the process involves shipping small containers filled with the reagents to a zinc processing station and then shipping empty containers back to the ammonia and/or carbon dioxide supplier. The cost of shipping a large amount of carbon dioxide and/or ammonia is lower, but this involves substantial investment in storage and handling facilities. The large amount also exceeds the requirement of the zinc processing facilities.
In accordance with the present invention, it is an object to provide a method for preparing zinc carbonate from zinc ash using a urea solution. A series of heating and filtering steps are used to remove various contaminating metals and to precipitate the final basic zinc carbonate product. Alternatively, rather than a series of heating steps, the pH of the solution is selectively adjusted to facilitate the removal of iron and to harvest the basic zinc carbonate.
In accordance with the present invention, a method is provided for producing basic zinc carbonate. The method includes heating a solution made from urea with added zinc ash in a vessel to produce zinc-ammonia-carbonate complex (Zn(NH3)xCO3, where x can be 2, 4 or 6 depending on the concentration of ammonia) and impurities dissolved in a solution and a residual. The solution is cemented by adding the zinc dust to form a slurry. The slurry is filtered to remove a first precipitate and to produce a first filtrate. The first filtrate is heated to form a second precipitate in the first filtrate. The first filtrate is filtered to remove the second precipitate and to produce a second filtrate. The second filtrate is heated to form a third precipitate comprising basic zinc carbonate. The basic zinc carbonate is harvested and washed.
In accordance with another aspect of the present invention, a method is provided for producing basic zinc carbonate. A solution made from urea with zinc ash is heated in a vessel to produce a zinc-ammonia-carbonate complex solution and impurities dissolved in the solution and a residual. The solution is cemented with zinc dust to form a slurry. The slurry is filtered to remove a first precipitate comprising heavy metals, some zinc value, and iron and to produce a first filtrate. The first filtrate is heated to form a second precipitate in the first filtrate. The first filtrate is filtered to remove the second precipitate comprising iron hydroxide and some zinc value to produce a second filtrate. The second filtrate is heated to form a third precipitate comprising basic zinc carbonate. The basic zinc carbonate is washed, dried, and milled.
In accordance with another aspect of the present invention, a method is provided for producing basic zinc carbonate. The method includes heating a solution made from urea with added zinc ash in a vessel from 90xc2x0 C. to 150xc2x0 C. to produce zinc-ammonia-carbonate complex and impurities dissolved in the solution and a residual. The solution is cemented with zinc dust to form a slurry. The slurry is filtered to remove a first precipitate and to produce a first filtrate. The pH of the first filtrate is adjusted to 10 or above to form a second precipitate comprising iron in the first filtrate. The first filtrate is filtered to remove the second precipitate and to produce a second filtrate. The pH of the second filtrate is adjusted to below 10 to form a third precipitate comprising zinc carbonate. The zinc carbonate is washed, dried, and milled.
A feature of the present invention concerns the use of urea in the preparation of zinc carbonate from zinc ash. An advantage to using urea rather than ammonia and carbon dioxide is that urea tends to be safer and easier to ship than ammonia and carbon dioxide and does not require special vehicles and equipment. Further, ammonia and carbon dioxide are transported in a dehydrated, solid form. In addition, urea is in a more concentrated form than ammonia and carbon dioxide. For example, urea has a molecular weight of 60 where one urea molecule can react with water to form two ammonia molecules and one carbon dioxide molecule for a total molecular weight of 78. Consequently, one can transport 60 parts solid urea instead of 78 parts compressed gasses (i.e., ammonia and carbon dioxide) which tend to be hazardous and expensive to ship.
An additional feature of the present invention concerns the reuse of ammonia and carbon dioxide evolved during the reaction of zinc ash with urea or the ammonia evolved during the precipitation of basic zinc carbonate by heating. These evolved gasses of ammonia and carbon dioxide are reintroduced to the zinc ash/urea solution. Additional external ammonia gas and/or carbon dioxide (i.e., gasses not evolved during the reaction of zinc ash with urea) may be added to the zinc ash/urea solution. An advantage of reintroducing ammonia and carbon dioxide to the zinc ash/urea solution is to conserve reagents and to further decompose the zinc ash.
Further features and advantages of the present invention will be set forth in, or apparent from, the,detailed description of preferred embodiments thereof which follows.